Same reason a $20 driver won't sound its best until its broken in. They are new and not broken in.
Break in time varies from driver to driver.
Who better to know how long one of these drivers in the design takes to break in than the ones who have been testing them ?

Just because you don't hear anything does not mean there is no difference, somebody who cannot hear a difference from one piece of gear from another may not hear obvious changes from a driver that is new to one that is broken in.
Take measurements if you can't hear the difference, that will tell you what you want to know.

Funeral was nice, Mom had lots of friends and relatives come from near and far.
Thank's to everybody for their kind gestures here on the forum.
The future will be different but memories of Mom will stay forever.

Mom died suddenly of heart failure last Sunday morning.
Bless her heart for dealing with Dad's and my refrigerator sized speakers, closets of records and C/D's for the past 50 + years and being a dam great mother.
Miss her dearly.
Obituary

Have a 50-60Hz hum/buzz in your system ? Its likely a ground loop issue and luckily there are ways to combat the issue.
Feel free to make this a sticky as the issue is common and not just related to subwoofers.

I have had 2 sets, the old set from late 80's and newer to me late 70's.
Both had bass bin resonance, luckily spent time on the refurbish on my newer to me set and added bracing to remove any signs of the irritating noise.

Adding mass like veneer may change the frequency of the resonance a bit but its not going to help with the problem.
Bracing will fix the issue, panel flex is proportional to the square of the unbraced span.

1/4" ply over the 3/4 will not stiffen the cabinet much at all, its just not the same as 1" material though you would think so.
Bracing is really the way to make it right, might be a bit more work but worth the time and be sure to brace the inside of the cabinet ( where the woofer is) as well so you have bracing all the way from one side to the other.

Cabinet size depends on driver T/S parameters.
You can't just shrink a cabinet without consequences.
Most blow so much money on speakers and amps that subs are an afterthought or have only this >[]< small space left for a sub and wonder why it does not sound that great in the end.